Power tool with signal generator

ABSTRACT

A power tool comprising a housing, a motor, an output shaft, a gear transmission mechanism and a control system. The gear transmission mechanism is connected between the motor and the output shaft to transmit the rotary power of the motor to the output shaft. The power tool comprises a signal generator and a control system electrically coupled to the signal generator and operatively engaged with the gear transmission mechanism such that when the signal generator is manually activated, an electric signal is generated and transmitted to the control system to cause the gear transmission mechanism to vary the gear reduction ratio.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(a)-(d) to ChinesePatent Application No. 200710131762.5, filed Aug. 29, 2007, and ChinesePatent Application No. 200810145304.1, filed Jul. 31, 2008. The contentsof these applications are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable speed power tool (such as anelectric drill, a screwdriver or an impact drill) having a signalgenerator which can be selectively activated to perform speed variation.

2. Description of the Related Art

A variable speed power tool (e.g., an electric drill, a screwdriver oran impact drill) usually employs a gear transmission system toselectively output various speeds to adapt to different work conditions.The gear transmission system generally includes a gear transmissionmechanism connected between an electric motor and an output shaft and acontrol system operable by the user to change the gear transmissionmechanism so as to vary the gear reduction ratio to output variousspeeds.

U.S. Pat. No. 6,431,289 discloses a gear transmission mechanismincluding two ring gears being adjustable forwards and backwards via anactuating member operated by the user outside the housing. By thismeans, the ring gears can shift between a rotating position where theyconnect both planet gears and an adjacent planet carrier and a fixedposition where they connect to the planet gears and the housing therebyvarying the output speed. However by such manual operation, the operatorhas to monitor the running bit (such as a fastening screw) to decidewhen to vary the output speed or torque to achieve better outputefficiency. This operation is not user-friendly. Furthermore, when theuser actuates the actuating member to shift the ring gear from therotating position to the fixed position, the rotating ring gear isunable to engage a fixed structure in the housing smoothly. In thesecircumstances, a gear clash between the ring gear and the rotationallyfixed structure frequently occurs which results in damage to the ringgear or the fixed structure and may reduce its working life.

U.S. Pat. No. 6,824,491 discloses an automatic variable speed system inwhich a control system is able to automatically vary the gear reductionratio by varying the direction of movement of the ring gear according tovariation of the torque load. EP-A-0787931 discloses a similar automaticvariable speed system. However, these automatic variable speed systemsare performed mechanically and the mechanical structure of the systemsare complex and difficult to manufacture. Neither system is able toeliminate the occurrence of gear clash.

JP-A-3221381 discloses a solenoid mounted on a motor which is able toactuate linear movement of a switching lever via an actuation lever anda transmitting lever to change the gear reduction ratio of the geartransmission mechanism.

JP-A-4262151 discloses a solenoid adapted to change the gear reductionratio of the gear transmission mechanism.

JP-A-8068461 discloses a keep solenoid used to drive an internal gearfrom a high speed position to a low speed position and retain theinternal gear at the low speed position. A spring is used to pull theinternal gear back to the high speed position.

JP-A-9057639 discloses an auto speed change mode which will not beactivated if the motor does not reach the highest output. It isundesirable that auto speed changing is actuated when the trigger switchis being depressed to increase the motor speed. A motor speed detectmodule is provided to detect motor speed and feed a signal representingmotor speed to the control circuit.

JP-A-9057640 discloses a mode selection switch provided to activate orinactivate an automatic speed changing function. The switch is used toselect the auto speed change mode and the fixed speed mode. The switchcan select between auto mode, high speed mode and low speed mode.

JP-A-10103462 discloses a curve ηh indicating the tool output efficiencyat a high speed state. ηl is a curve indicating the tool outputefficiency at a low speed state. The tool first works in the high speedstate and when the load torque reaches CP, the auto speed change isactivated so that the tool then works in the low speed state.

JP-A-9014433 discloses a keep solenoid which is used only to change fromhigh speed to low speed and various means for returning to high speed.

JP-A-1109845 discloses a battery pack voltage detector VP which willdetect BP voltage. When the voltage drops below a predetermined value,the auto speed mode is activated directly to vary from high speed to lowspeed. The activation of auto speed mode is not controlled by controlcircuit C. The control circuit C detects the load torque by monitoringthe motor speed. If the power drops rapidly and the motor speed reducessharply, the control circuit C cannot activate auto speed changingbefore the motor has stalled.

SUMMARY OF THE INVENTION

The present invention provides a variable speed power tool having asignal generator which can be activated to perform speed variation asrequired.

The present invention provides a power tool including a housing, amotor, an output shaft, a gear transmission mechanism and a controlsystem. The gear transmission mechanism is connected between the motorand the output shaft to transmit the rotary power of the motor to theoutput shaft at a gear reduction ratio. A signal generator iselectrically coupled to the control system. When the signal generator ismanually activated, an electric signal is generated and transmitted tothe control system and in response to the electric signal, the controlsystem causes the gear transmission mechanism to vary the reductionratio.

In one embodiment, the present invention provides a power tool capableof variable output speed comprising:

a housing;a motor contained in the housing for outputting rotary power;an output shaft;a gear transmission mechanism disposed between the motor and the outputshaft for transmission of the rotary power of the motor to the outputshaft at each of a plurality of gear reduction ratios;a signal generator; anda control system electrically coupled to the signal generator andoperatively engaged with the gear transmission mechanism such that whenthe signal generator is manually activated, an electric signal isgenerated and transmitted to the control system to cause the geartransmission mechanism to vary the gear reduction ratio.

Preferably, the signal generator comprises a switch mounted on thehousing.

In a preferred embodiment, the power tool comprises:

a handle coupled to the housing, wherein the handle is arranged at apredetermined distance from the switch such that the switch is operableby the hand of the user without removing the hand from the handle.

Preferably, the control system comprises:

a control unit electrically connected to the switch to receive theelectrical signal when the switch is activated and

a driving mechanism actuated by the control unit in response to theelectrical signal to cause the gear transmission mechanism to vary thegear reduction ratio.

Preferably, the gear transmission mechanism comprises:

at least one gear train and

a movable member movable between a first position corresponding to a lowgear reduction ratio and a second position corresponding to a high gearreduction ratio whereby to vary the gear reduction ratio.

Preferably, the gear train is a planetary gear train which includes aplurality of planet gears, an adjacent planet carrier, and arotationally fixed structure immovably associated with the housing.

Preferably, the movable member comprises a ring gear which when locatedat the first position engages the planet gears and the planet carrierand when located at the second position engages the planet gears and therotationally fixed structure.

Preferably, the driving mechanism is electromagnetically actuatable andoperatively engages the movable member, wherein the control unit isoperative to apply electric current to the driving mechanism to drivethe movable member.

In a particularly preferred embodiment, the electromagneticallyactuatable driving mechanism comprises:

a solenoid which includes a coil,

an iron core linearly movable through the coil and

a push bar attached to the iron core and connected to the movablemember.

Preferably, the control system comprises an H-bridge circuit for varyingthe direction of the electric current applied to the electromagneticallyactuatable drive mechanism.

Preferably, the control unit is capable of determining whether there isa load applied to the power tool before actuating the driving mechanism.

In a particularly preferred embodiment, the control unit is operative topause the motor for a first predetermined time interval if there is aload applied and to pause the motor for a second predetermined timeinterval if there is no load applied, wherein the first predeterminedtime interval is shorter than the second predetermined time interval.

In a preferred embodiment, the plurality of gear reduction ratiosincludes a low gear reduction ratio and a high gear reduction ratio andthe gear transmission mechanism comprises:

at least one gear train and

a movable member variably engaged with the gear train, wherein themovable member is movable between a first position corresponding to thelow gear reduction ratio and a second position corresponding to the highgear reduction ratio, wherein the movable member has a rotational speedat the first position and the control system is operatively associatedwith the movable member and is capable of determining an operatingcharacteristic indicative of a load applied to the output shaft, whereinthe control system is operative to actuate the movable member to movefrom the first position to the second position and to reduce therotational speed of the movable member when the operating characteristicreaches or exceeds a predetermined value.

Preferably, the control system is operative to firstly reduce therotational speed of the movable member and to then actuate the movablemember to move from the first position to the second position.

Preferably, the control system is operative to reduce the rotationalspeed of the movable member to zero.

Preferably, the control unit is capable of determining the operatingcharacteristic indicative of a load applied to the output shaft andreducing the rotational speed of the movable member.

Preferably, the power tool further comprises a driving mechanismactuatable by the control unit to drive the movable member between thefirst position and the second position.

The driving mechanism may be, or comprise, a small motor, a linearmotor, a servo motor or a piezoelectric motor to assist in performingautomatic speed variation.

In a particularly preferred embodiment, the driving mechanism iselectromagnetically actuatable and operatively engages the movablemember, wherein the control unit is operative to apply electric currentto the driving mechanism to drive the movable member.

In a particularly preferred embodiment, the driving mechanism comprises:

a solenoid which includes a coil, an iron core linearly movable throughthe coil and a push bar attached to the iron core and connected to themovable member.

The operating characteristic may be an electrical parameter (such aselectric current or voltage of the motor), rotational speed or torque ofthe motor or output shaft or stress of the mechanical components.

In a preferred embodiment, the operating characteristic is the motorcurrent.

In a further embodiment, the present invention provides a power tool forvarying output speed, comprising:

-   -   a housing,    -   a motor contained in the housing for outputting rotary power;    -   an output shaft;    -   a gear transmission mechanism disposed between the motor and the        output shaft for transmission of the rotary power of the motor        to the output shaft at each of a plurality of gear reduction        ratios including a first gear reduction ratio and a second gear        reduction ratio, the gear transmission mechanism comprising        -   at least one gear train and        -   a movable member variably engaged with the gear train,            wherein the movable member is movable between a first            position corresponding to the first gear reduction ratio and            a second position corresponding to the second gear reduction            ratio; and    -   a control system comprising a mechanism for driving the movable        member to move from the first position to the second position        and from the second position to the first position.

Preferably, the mechanism comprises a bi-directional keep solenoidcapable of retaining the movable member at the first or the secondposition without electricity.

In a particularly preferred embodiment, the bi-directional keep solenoidcomprises a push bar connected to the movable member.

In a particularly preferred embodiment, the bi-directional keep solenoidfurther comprises:

a longitudinal metallic shell,

a pair of coils arranged longitudinally in the shell,

a permanent magnet disposed between the pair of coils and

an iron core linearly movable through the pair of coils, wherein thepush bar is fixed to the iron core.

Preferably, the gear train is a planetary gear train which includes aplurality of planet gears, an adjacent planet carrier, and arotationally fixed structure immovably associated with the housing.

Preferably, the movable member comprises:

a ring gear which when located at the first position engages the planetgears and the planet carrier and when located at the second positionengages the planet gears and the rotationally fixed structure.

Preferably, the control system further comprises:

a control unit operative to apply electric current to the mechanism todrive the movable member.

Preferably, the control system comprises an H-bridge circuit forchanging the direction of the electric current applied to the mechanism.

The power supply may be a DC power supply (preferably a rechargeablebattery pack). The battery pack may be composed of a number of seriallyconnected cells. The cells can rely on any cell chemistry such as forexample lead-acid, Nickel-cadmium (“NiCd”), Nickel-Metal Hydride(“NiMH”) or Lithium-based chemistry such as Lithium-cobalt (“Li—Co”) orLithium-manganese (“Li—Mn”). The battery pack may have a nominal voltagesuch as 14.4V, 18V or 21V dependent on the number of cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become more apparent from the following description of embodimentsin conjunction with the accompanying drawings in which:

FIG. 1 is an elevational side view of a variable speed power tool of afirst embodiment of the present invention, wherein the housing of thepower tool is partially cutaway to show a gear transmission mechanism;

FIG. 2 is a partial enlarged view of the driving mechanism of the powertool of FIG. 1;

FIG. 3 a is a partial enlarged view of the gear transmission mechanismof the power tool of FIG. 1 in a high speed state;

FIG. 3 b is a partial enlarged view of the gear transmission mechanismof the power tool of FIG. 1 in a low speed state;

FIG. 4 a is an elevational side view of a variable speed power tool of asecond embodiment of the present invention, wherein the housing of thepower tool is partially cutaway to show a gear transmission mechanism;

FIG. 4 b is a partial enlarged view of the driving mechanism of thepower tool of FIG. 4 a;

FIG. 5 is a block diagram of a first electronic control system forcontrolling the gear transmission mechanism;

FIG. 6 is a block diagram of a second electronic control system forcontrolling the gear transmission mechanism which further includes apower switch;

FIG. 7 is a block diagram of a third electronic control system forcontrolling the gear transmission mechanism which further includes afield effect transistor;

FIG. 8 is a block diagram of a fourth electronic control system forcontrolling the gear transmission mechanism which further includes aclutch;

FIG. 9 is a graph of the output efficiency η of the power tool versusthe torque load T applied to the power tool to illustrate the timing ofswitching from high speed to low speed;

FIG. 10 illustrates an elevational side view of a variable speed powertool of a third embodiment of the present invention with a drivingmechanism in a high speed state;

FIG. 11 illustrates the driving mechanism of FIG. 10 in a low speedstate;

FIG. 12 is a partial cross-sectional view of the power tool taken alongthe line 12-12 of FIG. 11;

FIG. 13 is a partial cross-sectional view of the power tool taken alongthe line 13-13 of FIG. 11;

FIG. 14 is a cross-sectional view of a bi-directional keep solenoid ofthe power tool of FIG. 10;

FIG. 15 is an elevational side view of a power tool of a fourthembodiment of the present invention with a signal generator disposed onthe housing;

FIG. 16 is a top view of the power tool of FIG. 15 which additionallyshows a side handle and a speed mode selector with a button adjustablebetween three positions;

FIGS. 17 a and 17 b show two forms of an activation switch of the signalgenerator of FIG. 15;

FIG. 18 is a top view of a power tool of a fifth embodiment of thepresent invention with a speed mode selector with two positions;

FIG. 19 is a top view of a power tool of a sixth embodiment of thepresent invention with two LEDs representative of high speed and lowspeed;

FIG. 20 is a simplified circuit diagram of a preferred embodiment of thepower tool of the invention showing the electronic control systemconnected to the motor and the power supply;

FIG. 21 is a flowchart illustrating an operation of the electroniccontrol system for automatic speed variation;

FIG. 22 is a flowchart illustrating an operation of the electroniccontrol system for one touch speed variation;

FIGS. 23 a and 23 b shows a power tool of a seventh embodiment of thepresent invention and its driving mechanism at high speed and low speedrespectively; and

FIG. 24 is a partial cross-sectional view of the power tool taken alongthe line 24-24 of FIG. 23 a.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 3 b show a variable speed power tool in accordance with afirst embodiment of the present invention. The power tool 9 includes anelectric motor 2, a power supply 1, a main switch 13 for starting orstopping the motor 2, an output shaft 6 and a gear transmissionmechanism 4. The gear transmission mechanism 4 includes a firstplanetary gear train including a plurality of first planet gears 40 anda first planet carrier 41, a second planetary gear train including aplurality of second planet gears 42 and a second planet carrier 43, arotationally fixed structure 44 fixed inside a housing 21 and an axiallymovable member 45. A driving mechanism 5 engages the gear transmissionmechanism 4 and includes an actuating device 52 and a transmissionmember 51.

In the first embodiment, the actuating device 52 is electromagnetic andincludes a pair of spaced apart permanent magnets 521, an iron core 523located between the permanent magnets 521 and a coil 522 around the ironcore 523. In the middle of the iron core 523 is defined a groove 524.The transmission member 51 includes an arc-shaped push bar 512 and asteel wire 513 engaged with the push bar 512. The push bar 512 has anengaging portion which is capable of projecting into the groove 524 ofthe iron core 523. In this embodiment, the movable member 45 is a ringgear having internal teeth 451 and side teeth 452. The movable member 45defines an annular groove 453 for accommodating the steel wire 513.

When electric current flows through the actuating device 52, the ironcore 523 is magnetized and attracted by one of the permanent magnets521. When the direction of electric current is reversed, the iron core523 is attracted by and moved to the other of the permanent magnets 521and the push bar 512, steel wire 513 and movable member 45 are actuatedto move along with the iron core 523.

FIG. 3 a shows the gear transmission mechanism 4 in a high speedposition in which the iron core 523 is attracted by one of the permanentmagnets 521. Accordingly the internal teeth 451 of the movable member 45mesh with external teeth 411, 421 of the first planet carrier 41 and thesecond planet gears 42. By this engagement, the gear transmissionmechanism 4 has a relatively low gear reduction ratio and the outputshaft 6 attains a relatively high speed.

FIG. 3 b shows the gear transmission mechanism 4 in a low speed positionin which the iron core 523 is attracted by the other of the twopermanent magnets 521. Accordingly the internal teeth 451 of the movablemember 45 mesh with the external teeth 421 of the second planet gears 42only. Meanwhile, the side teeth 452 of the movable member 45 mesh withthe teeth 441 of the rotationally fixed structure 44 to be fixed withrespect to the housing. By this engagement, the gear transmissionmechanism 4 has a relatively high gear reduction ratio and the outputshaft 6 attains a relatively low speed.

FIGS. 4 a and 4 b show a second embodiment of a power tool 9 of theinvention and its driving mechanism 5. The driving mechanism 5 iselectromagnetic. When electric current flows, the movable member 45 isactuated to move between a high speed position and a low speed position.In this second embodiment, the actuating device 52 is a servo motor 525and the transmission member 51 includes a screw member 515, a push bar512 with inner threads engaging the screw member 515 and a steel wire513. The servo motor is 525 able to rotate in a forward direction and areverse direction to actuate the push bar 512 together with the movablemember 45 to move along the screw member 515 between the high speedposition and the low speed position.

FIG. 5 is a block diagram of a first electronic control system 8 of thepower tool 9 of the invention. The electronic control system 8 includesan electronic control unit 3 and the driving mechanism 5. The electroniccontrol unit 3 includes a microcontroller (MCU) 30 and an input/outputcircuit. A signal generator 35 is connected to the microcontroller 30. Aspeed mode selector 36 is connected between the microcontroller 30 andthe power supply 1. The driving mechanism 5 is controlled by themicrocontroller 30 to perform automatic speed variation. Aftercompleting speed variation, the movable member 45 arrives at the high orlow speed position as required. In the meantime, the microcontroller 30receives a position feedback signal indicating the current position ofthe movable member 45. The operator can selectively activate orinactivate the function of automatic speed variation by operating thespeed mode selector 36. The form of the speed mode selector will bedescribed in more detail hereinafter with reference to FIGS. 16 and 18.

In auto mode, the microcontroller 30 determines an operatingcharacteristic of the power tool 9 and compares it with a predeterminedvalue to then determine whether to activate automatic speed variation.It will be apparent to those skilled in the art that an operationalamplifier circuit may be used in place of the microcontroller 30 toperform the comparison step. The operating characteristic may be anelectrical parameter (such as electric current or voltage of the motor),rotational speed or torque of the motor or output shaft or stress of themechanical components.

In some circumstances, even in the auto mode, the operator may wish toself-determine the speed variation whilst the power is still running. Inthis case, the operator can input a control signal to themicrocontroller 30 via a signal generator 35. In response to the controlsignal, the microcontroller 30 performs the operation for speedvariation immediately.

During the course of automatic speed variation, the electronic controlunit 3 may issue a control signal to actuate the driving mechanism 5.The actuating device 52 of the driving mechanism 5 then shifts thetransmission member 51 along with the movable member 45 from the highspeed position to the low speed position. During this period, themovable member 45 (the ring gear in this embodiment) is required tochange from a rotational running state to a standing state so that it isable to mesh with the fixed structure smoothly. Otherwise, a gear clashbetween the movable member 45 and the rotationally fixed structure 44occurs. In order to avoid gear clash, one of three electronic controlunits discussed in detail below may be deployed.

FIG. 6 shows a block diagram of a second electronic control system 8comprising the electronic control unit 3 and the driving mechanism 5.The control unit 3 includes the microcontroller 30, a speed modulatingcircuit 31, a load detecting circuit 32, a driving mechanism controlunit 33 and a position feedback circuit 34. A power switch 10 such asMOSFET, triac or relay is connected between the power supply 1 and themotor 2. The power switch 10 is controlled by the speed modulatingcircuit 31.

At start-up of the power tool 9, the microcontroller 30 closes the powerswitch 10 after a predetermined delay (for example 0.1 seconds). In theperiod of delay, the microcontroller 30 sends an instruction signal tothe driving mechanism control circuit 33 for resetting the drivingmechanism 5 to the high speed position. The power supply 1 then powerson the motor 2 and at this time, the output shaft 6 begins to drive atool bit (not shown) applied to a work piece (not shown). The loaddetecting circuit 32 samples the electric current flowing through themotor 2 in real time. The microcontroller 30 then compares the currentvalue with a predetermined value. If the current value is larger thanthe predetermined value for a predetermined time period, the power tool9 is overloaded and it is required to switch the output speed to arelatively low state. The microcontroller 30 then sends an instructionsignal to the speed modulating circuit 31 to control the power switch 10to cut off the power supply to the motor 2 thereby reducing therotational speed of the motor 2 to zero. The rotational speed of thering gear 45 is also reduced to zero. The microcontroller 30 then issuesan instruction signal to the driving mechanism control circuit 33 toreverse the electric current. After the current has reversed, the ironcore 523 along with the movable member 45 is moved from the high speedposition to the low speed position. Since the rotation of the movablemember 45 has been reduced to zero before starting to move, the movablemember 45 is capable of engaging the rotationally fixed structuresmoothly and does not cause gear clash.

After the iron core 523 reaches the other of the permanent magnets 521,the position feedback circuit 34 of the electronic control unit 3receives a position signal indicating the present position of the ironcore 523 and the movable member 45 and passes the signal to themicrocontroller 30. It will be apparent to a person skilled in the artthat the position signal may be issued by detecting the position of themovable member 45. The microcontroller 30 then sends an instructionsignal to the speed modulating circuit 31 to turn on the power switch 10and the power supply 1 provides power to the motor 2. Thus the motoroutputs a low speed and large torque.

When the variable power tool 9 works at high speed, the motor currentmay suddenly increase due to troublesome operation. The cause of thismay be an irregularity of the threaded groove on the bolt or a slightwarp of a member interleaved between the bolt and the nut or clipping ofdust between the bolt and the nut. There is a risk that the automaticspeed variation is undesirably actuated due to the current momentarilyexceeding a predetermined threshold value. To avoid this possibility,the comparison of the sampled current with the threshold value lasts fora predetermined time period (eg 0.5 seconds), Only when the sampledcurrent remains larger than the predetermined threshold value beyond thetime period will the automatic speed variation be actuated.

The operator can deploy the speed mode selector to select a manual modeto vary the output speed. The preset threshold value is determined onthe upper current value when the power tool is the high speed state. Inthe present embodiment, the preset threshold value is 30 A. If theoperator wishes to initiate automatic speed variation before the motorcurrent value reaches the predetermined threshold value, a controlsignal to the microcontroller 30 can be inputted via the signalgenerator 35. The microcontroller 30 then initiates speed variationimmediately. Alternatively, the preset threshold value may be determinedby a number of historical values that the operator has inputtedpreviously. In other words, the electronic control unit 3 is able to“learn” itself. To achieve this, the electronic control unit 3 may storethe values of the motor current at the time when the operator inputs thecontrol signal to activate the speed variation and the control unit mayset the mean of the stored values as the threshold value.

In the above mentioned embodiment, the microcontroller 30 stops themotor 2 before the movable member 45 meshes with the rotationally fixedstructure so as to avoid gear clash. However if the rotational speed ofthe motor 2 reduces to a relatively low level, gear clash can still beavoided when the movable member 45 meshes with the rotationally fixedstructure 44.

FIG. 7 shows a third electronic control system comprising the electroniccontrol unit 3 and the driving mechanism 5. In the main circuit, thepower switch 10 is replaced by a field effect transistor 11 (for examplea MOSFET). When automatic speed variation is activated, the speedmodulating circuit 31 controls the field effect transistor 11 to reducethe rotational speed of the motor 2 so that the movable member 45 has aspeed reduced to a relatively low level such that no gear clash occurswhen it meshes with the rotationally fixed structure 44.

FIG. 8 is a block diagram of a fourth electronic control systemcomprising the electronic control unit 3 and the driving mechanism 5. Aclutch mechanism 12 is provided between the motor 2 and the geartransmission mechanism 4. Similar to the third electronic controlsystem, the speed modulating circuit 31 controls the clutch mechanism 4to reduce the torque transmitted from the motor 2 to the geartransmission mechanism 4 thereby reducing the rotational speed of themovable member 45 to a relatively low level before the movable member 45engages the rotationally fixed structure. As described hereinbefore, theelectronic control unit 3 detects whether the load applied to the powertool 9 exceeds the predetermined threshold value after a predeterminedtime period and then if necessary, modulates the speed of the movablemember 45 or gear transmission mechanism 4 via the speed modulatingcircuit 31. When the rotational speed of the movable member 45 issufficiently low, the driving mechanism 5 is activated to automaticallyperform speed variation without gear clash.

After automatic speed variation is complete, the power supply to themotor 2 is resumed. If the rotational speed of the motor increases fromzero to a normal value instantly, a large torque will be generated whichmay result in slippage of the tool from the hands of the operator. Thiscan be avoided by using the field effect transistor 11 to soft start themotor 2. Specifically, a time interval of typically 0.6 s exists betweenthe motor 2 beginning to rotate and the motor 2 working at a normalworking state. During the interval, the electronic control unit 3controls the field effect transistor 11 by varying a pulse widthmodulating signal (for example by adjusting the pulse width in a fixedfrequency or adjusting the pulse frequency in a fixed width) in such amanner as to step up the voltage applied to the motor 2 to the normalworking state. This lessens the impact on the operator.

An alternative timing for switching the output speed of the power toolwill be described with reference to FIG. 9. FIG. 9 is a graph of theoutput efficiency of a power tool versus the torque load T applied tothe output shaft. The output efficiency of the power tool is the ratioof the output power of the output shaft to the input power of the motor.Curve ηH represents the output efficiency of the tool running at highspeed and curve ηL represents the output efficiency of the tool runningat low speed. As shown in FIG. 9, before the torque load value reachesTc, the value of the output efficiency of the tool at high speed ishigher than that at low speed. When the torque load value exceeds Tc,the value of the output efficiency of the tool at high speed will belower than that at low speed. Therefore to ensure that the power toolalways performs at a high output efficiency, it is supposed that thepower tool runs at high speed before the torque load value reaches Tcand the power tool turns to run at low speed after the torque load valueexceeds Tc. In other words, the best time for switching the speed fromhigh to low is when the torque load value equals Tc.

The current flowing through the motor running at high speed is employedas an operating characteristic of the power tool to indicate the loadapplied to the output shaft. Referring to FIG. 9, the current curve IHmay rise linearly as the torque load T increases. When the torque loadreaches Tc, the curve ηH intersects the curve 1L at the transition pointηC and the motor current increases to a switching point Ic. In thepresent embodiment the value of Ic is 30 A. A person skilled in the artwill recognize that a resistor (383 in FIG. 20) may be connected inseries with the motor to act as a current sensor. A microcontroller (30in FIG. 20) may connect to the current sensor to monitor the motorcurrent. When the microcontroller detects that the motor current reachesIc, it executes an algorithm to automatically switch from high speed tolow speed. The operation of the microcontroller will be described ingreater detail hereinafter in connection with FIG. 21. It will be notedthat the motor current curve may be different for various types of motorbut generally the rise of the curve may be linear or substantiallylinear. The present embodiment employs the motor current to representthe load applied to the output shaft. As stated in previous embodiments,a person skilled in the art will recognize that a variety of operatingcharacteristics of the power tool can provide the same effect (such asthe torque or speed of the output shaft, the speed of the motor and thetorque or speed of gears of the gear transmission mechanism). When usinga battery pack for power supply, voltage variation to the contacts ofthe battery pack may also be used to represent the applied load.

The power tool according to a third embodiment of the present inventionwill be described with reference to FIGS. 10-14. In the thirdembodiment, the driving mechanism 5 is electromagnetic and comprises abi-directional keep solenoid 53 capable of retaining the iron core atopposite ends of the travel of the iron core without electricity.Referring to FIG. 14, the bi-directional keep solenoid 53 includes alongitudinal magnetized metallic shell 531, a pair of coils 532 arrangedlongitudinally in the shell 531, a permanent magnet 533 disposed betweenthe pair of coils 532, an iron core 534 linearly movable through thecircled areas of the pair of coils 532 and a push bar 535 fixed to theiron core 534. The push bar 535 is provided near an end with acircumferential groove 536 exposed outside the shell 531. FIG. 14 showsthat the push bar 535 is located at a rearward position and the ironcore 534 is retained at this position due to attraction of the permanentmagnet 533 even when no electric current flows through the coils 532.Referring to FIGS. 10 and 12, a connection member 54 is disposed betweenthe steel wire 513 and the push bar 535 which includes a supporting arm541 generally surrounding a lower half of the gear case 47 and a pair ofspaced side plates 542 extending downward from the bottom of thesupporting arm 541. The supporting arm 541 has two free ends connectedto opposite radially extending ends of the steel wire 513. The sideplates 542 extend rearwardly and engage the groove 536 to connect to thepush bar 535. The bi-directional keep solenoid 53 is accommodated in thespace between the pair of side plates 542. Referring to FIG. 12, aguiding mechanism is disposed between the side plates 542 and thehousing 21. The guiding mechanism includes a pair of guide bars 543extending laterally from each side plate 542 and two pairs of ribs 544each extending laterally from each side of the housing 21 to define aguiding groove 545 for receiving and guiding the corresponding guide bar543.

FIG. 10 shows that the push bar 535 of the bi-directional keep solenoid53 is located at a forward position where it extends outside the shell531 with a longer distance, whilst the iron core 534 is retained by thepermanent magnet 533. In this position, the gear transmission mechanism4 has a lower gear reduction ratio (ie the output shaft 6 or the powertool 9 is running at a high speed). At this time, the ring gear 45engages the planet carrier 41 and planet gears 42 to rotate together.Referring to FIG. 14, when a forward electric current flows through thecoils 532, a magnetic field is created to actuate the iron core 534 andthe push bar 535 to move from the forward position to the rearwardposition. During this period, the connection member 54, the steel wire513 and the ring gear 45 move together with the push bar 535 over adistance d (see FIG. 10). As the push bar 535 arrives at the rearwardposition, the ring gear 45 disengages the planet carrier 41 whilst stillengaging the planet gears 42. In the meantime, the ring gear 45 engagesthe rotationally fixed structure 44 to be immovable with respect to thehousing. Consequently, the gear reduction ratio increases whilst theoutput speed of the power tool decreases. If a reverse electric currentis applied to the solenoid 53, the push bar 535 returns to the forwardposition and the gear reduction ratio decreases whilst the output speedof the power tool 9 increases.

The signal generator 35 of the power tool of a fourth embodiment of thepresent invention will be described with reference to FIGS. 15-17. Asdescribed above, when the signal generator 35 is activated, theelectronic control system changes the direction of the electric currentflowing through the driving mechanism 5 thereby activating the speedvariation. Referring to FIG. 16, in conjunction with FIGS. 17 a and 17b, the signal generator 35 includes a pair of switches disposed onrespective halves of the housing 21. Each switch can activate the speedvariation. By this arrangement, a left-handed operator and aright-handed operator can conveniently activate speed variation by asingle touch of the switch.

FIGS. 17 a and 17 b show two forms of the switch. As shown in FIG. 17 a,a pushbutton switch 351 is mounted on the housing 21 and a spring plate353 is mounted on the housing spaced apart and covering the pushbuttonswitch 351. If the operator wishes to vary the output speed of therunning power tool 9, the spring plate 353 is depressed and presses apushbutton 3511 to close the switch 351. An electrical signal is issuedand outputted to the microcontroller 30 through wires 3512 to activatethe speed variation (as described hereinafter). Similarly, FIG. 17 bshows a spring switch 352 having a spring button 3521, contacts 3523 andwires 3522 electrically connecting the contacts to the microcontroller30. As the spring plate 353 is depressed, the spring button 3521 ispressed to contact the contact 3523 and the electrical signal isgenerated and outputted via the wires 3522.

FIG. 16 shows a speed mode selector 36 for selectively activating ordeactivating the automatic speed variation mode and selecting betweenhigh speed mode (H) and low speed mode (L) when in the deactivatedstate. The speed mode selector 36 includes a button 361 exposed on thehousing 21 and a number of terminals electrically connected toelectronic components (described hereinafter). The button 361 can beoperated to slide on the housing 21 between three positions, namely automode position (A), high speed mode position (H) and low speed modeposition (L). The terminals include three signal terminals 362corresponding to above three mode positions and a ground terminal 363.The operation of the speed mode selector 36 will be described in detailhereinafter with reference to FIG. 20.

When the speed mode selector 36 is located at the auto mode position (A)shown in FIG. 16, the automatic speed variation is activated and cannotbe interrupted. Even if the switch 351 or 352 is closed, the functionachieved by the signal generator 35 would not be activated. Only whenthe button 361 of the speed mode selector 36 is adjusted to the highspeed position (H) or the low speed position (L) can the activation ofthe signal generator 35 achieve speed variation.

Referring to FIG. 16, a side handle 221 is mounted on the housing 21.The side handle 221 is arranged at a predetermined distance 1 from thesignal generator 35 where the switches 351, 352 are positioned in such away that the signal generator 35 can be operated by the hand of theoperator without needing to take the hand away from the handle. It willbe apparent to those skilled in the art that the signal generator 35 canbe arranged at a predetermined distance 1 from the main handle 23 suchthat the user can operate the signal generator 35 with the hand holdingthe main handle 23.

FIGS. 18 and 19 illustrate fifth and sixth embodiments of a power toolof the invention with alternative combinations of the signal generatorand the speed mode selector. In the fifth embodiment in FIG. 18, thespeed mode selector 36 can be switched between two speed positions only,namely a high speed position H and a low speed position L. When thebutton 361 is located at the high speed position H, activation of thesignal generator 35 can switch the movable member to the low speedposition L. Similarly, activation of the signal generator 35 can switchthe movable member 45 from the low speed position L to the high speedposition H.

The power tool of the sixth embodiment is not equipped with a speed modeselector button. Instead, two light emitting diodes (LED) 371, 372 aredisposed on the housing 21 for indicating the high speed position andthe low speed position respectively. In this embodiment, the power toolstarts at a defaulted auto mode. On activating the signal generator 35,the auto mode is interrupted and the speed position will be switched.For example, the power tool initially runs in the auto mode at highspeed and the H-LED 371 is illuminated to represent the high speedstate. As the signal generator 35 is activated, the auto mode isinterrupted and the speed state is switched to and maintained at lowspeed until the signal generator 35 is activated again or the powersupplied to the motor 2 varies. If the signal generator 35 is activatedagain, the high speed state is resumed and maintained.

FIG. 20 is a simplified circuit diagram of the control system of apreferred embodiment of the present invention. The control system isconnectable to the power supply 1 via the main switch 13 (S1). In thepresent embodiment, the cells have a lithium-based chemistry and thebattery pack has a nominal voltage of approximately 18V.

Referring to FIG. 20 in conjunction with FIG. 6, the electronic controlsystem 8 includes a microcontroller 30, a speed modulating circuit 31, aload detecting circuit 32, a driving mechanism control circuit 33, aposition feed back circuit 34, a signal generator 35 and a speed modeselector 36.

The microcontroller 30 is a microcomputer with its memory andInput/Output (I/O) integrated into a single chip. The microcomputer is acomputer built using a microprocessor and other components for thememory and I/O. The microprocessor generally refers to theimplementation of the central processor unit functions of a computer ina single, large scale integrated circuit. The microcontroller typicallyincludes a central processing unit (CPU), a read-only memory (ROM), arandom access memory (RAM), a timer, a digital-to-analog (A/D) converterand a number of input/output ports P1-P28.

As shown in FIG. 20, the main switch 13 controls the battery packvoltage across a DC/DC converter 381 to be converted to a relative lowconstant voltage that serves as the power source for the control system.In the present embodiment, the constant voltage value is 5V. As the mainswitch 13 is closed, the constant voltage is supplied to themicrocontroller 30 via the port P20. A signal is outputted forinitializing the microcontroller 30 to the input port P1. The mainswitch 13 may be a potentiometer to which is coupled a trigger button131 (see FIG. 10). As the trigger button 131 is depressed, thepotentiometer 13 provides a signal in accordance with the degree of thedepression to the microcontroller 30 via the input port P2. Themicrocontroller 30 then outputs a signal through the output port P12 tothe speed modulating circuit 31 so as to control the power switch 10 forcontrolling the voltage applied across the motor 2. The speed modulatingcircuit 31 may be a metallic oxide semiconductor field effect transistor(MOSFET) drive circuit and the power switch 10 may be a MOSFET. Themicrocontroller 30 may output a pulse width modulated (PWM) controlsignal dependent on the potentiometer signal received at port P2 to theMOSFET drive circuit 31 and the circuit 31 varies the duty cycle of thePWM control signal. For example, as the trigger button 131 is moredeeply depressed, the circuit 31 increases an “on-time” duration of eachcycle of the PWM control signal in response to the current position ofthe trigger button 131. Thus a larger current flows through the motor 2to increase its rotational speed. It will be apparent to the skilledperson in the art that the MOSFET drive circuit may also vary the dutycycle frequency of the PWM control signal to vary the speed of the motor2.

The microcontroller 30 is designed to detect the battery pack voltage atport P3. The microcontroller 30 may read the signal received at port P3indicative of the residual capacity of the battery pack 1. The signalmay be a voltage signal. The microcontroller may then compare it with aselected threshold value to judge whether the battery pack isover-discharged. If the battery pack 1 is over-discharged, themicrocontroller 30 may process an over-discharge protection program suchas blocking the output port P12 to interrupt the power supplied to themotor 2. As shown in FIG. 20, four LEDs D2, D3, D4, D5 are connected tothe ports P18, P17, P16, P15 via resistors R22, R23, R29, R30respectively. These LEDs may be disposed on the housing, on the handleor at other areas of the power tool visible to the operator. These LEDsmay emit light in different combinations or in different colors toindicate different residual capacity levels of the battery pack 1. TheLEDs D2, D3, D4 emit green light whilst the LED D5 emits red light.

LEDs D2, D3 and D4 are used to show that the residual capacity of thebattery pack is at a normal level, LED D5 is used to show that theresidual capacity of the battery pack is at a lower level or in anover-discharge state. When the residual capacity is full, LEDs D2, D3and D4 are illuminated constantly whilst LED D5 is non-illuminated. Whenthe residual capacity is at a relatively high level, LEDs D3 and D4 areconstantly illuminated whilst LEDs D2 and D5 are non-illuminated. Whenthe residual capacity is at a middle level, LED D4 is constantlyilluminated whilst LED D2, D3 and D5 are non-illuminated. When theresidual capacity is at a relatively low level, LED D2, D3 and D4 arenon-illuminated whilst LED D5 is constantly illuminated. As the batterypack is over-discharged, LED D2, D3 and D4 are non-illuminated, whileLED D5 blinks. In the mean time, the microcontroller 30 controls theMOSFET drive circuit to interrupt the power supplied to the motor 2.

As stated above, the resistor 383 is connected in series with the motor22 and the power switch 10 to be used by the microcontroller 30 todetect the motor current indicative of a load applied to the power tool9. The load detecting circuit 32 is connected between the resistor 383and the microcontroller 30. The load detecting circuit 32 is anamplifying circuit which includes an operational amplifier 382 andresistors R8, R9 for determining an amplifying ratio. The operationalamplifier 382 amplifies the voltage drop caused by the resistor 383 atthe predetermined amplifying ratio and outputs a signal to the port P4.The microcontroller 30 reads the signal and determines whether thecurrent flow through the motor reaches the switching point Ic (see FIG.9). In addition, an over-current detecting circuit 37 is also connectedbetween the resistor 383 and the microcontroller 30. The over-currentdetecting circuit 37 may include a comparator 384 and a resistor (notshown) connected between the constant voltage supply (+5V) and thecomparator 384 to provide a reference voltage to one input (+) of thecomparator 384. The comparator 384 receives a voltage signal indicativeof the voltage drop caused by the resistor 383 at the other input (−)thereof and compares it with the reference voltage to determine whetherthe motor is over-current. The comparator 384 then outputs a high statesignal or low state signal indicative of non over-current orover-current respectively to the port P21 through resistor R24. Themicrocontroller 30 then executes an over-current protection algorithmwhich will be described hereinafter. In addition, a temperature sensingdevice 385 is coupled between the motor 22 and the microcontroller 30.The temperature sensing device may be a negative temperatureco-efficient (NTC) resistor R12 or thermistor adhered to the motorsurface. The resistance value of R12 varies with the motor temperature.A circuit composed of the resistor R10 and R12 can be used to provide ananalog signal representative of the voltage drop caused by the resistor385 to the microcontroller 30 through the port P5. The microcontroller30 may receive and interrupt the analog signal into a digital signal toindicate the motor temperature and then compare it with a preselectedthreshold value to determine whether the motor is over-temperature.

The battery pack 1 includes an identification resistor 386 (Ri)connected to the microcontroller 30 via input port P7 to indicate thetype or capacity of the battery pack. After the battery pack is attachedto the power tool, the resistor Ri together with the resistors R13 andR21 form a voltage divider circuit. The microcontroller 30 receives viaport P7 an analog signal which can be interpreted by the microcontroller30 to identify the type or capacity of the battery pack. Themicrocontroller 30 then determines a corresponding dischargingprotection program to avoid over-discharging.

The driving mechanism control circuit 33 is connected between thedriving mechanism 5 and the microcontroller 30 and between the powersupply 1 and the ground. The driving mechanism control circuit 33 is anH-bridge circuit and the driving mechanism is a bi-directional keepsolenoid 53. The H-bridge circuit includes four input ports A, B, C, Dconnected to the ports P11, P9, P10, P6 respectively and foursemiconductor switches Q1, Q2, Q3, Q4 connected to the ports B, C, D, Avia resistors R1, R2, R3, R4 respectively. The bi-directional keepsolenoid 53 is connected between an H node between the semiconductorswitches Q1, Q2 and an L node between the semiconductor switches Q3, Q4.The semiconductor switches Q1, Q2, Q3, Q4 are MOSFET (although otherswitches may be used). The microcontroller 30 controls the open orclosed state of the input ports A, B, C, D of the H-bridge circuit tovary the movement direction of the iron core 534 or push bar 535 of thesolenoid 53. For example, as the input ports A, B are closed while theinput ports C, D are open, the electric current supplied by the powersource 1 sequentially flows through the switch Q1, the H node, thesolenoid 53, the L node, the switch Q4 and ground. In other words, theelectric current applied to the solenoid 53 is in the direction of H toL. On the contrary, if the input ports A, B are open while the inputports C, D are closed, the electric current supplied by the power source1 sequentially flows through the switch Q3, the L node, the solenoid 53,the H node, the switch Q2 and ground. In other words, the electriccurrent applied to the solenoid 53 is in the direction of L to H. Byvarying the current applied to the solenoid, the push bar 535 is able tomove the ring gear 45 from the high speed position to the low speedposition or from the low speed position to the high speed position.

The position feed back circuit 34 is connected to microcontroller 30 andincludes a single-pole double-throw switch 387 (S5) and resistors R25′,26, R27, R28. As shown in FIG. 20, the switch 387 has two contacts L1,H1. In this case, the pole contacts the contact L1 which means the ringgear 45 is located at the low speed position. The switch 387 outputs alow state signal through resistor R27 to the port P13 of themicrocontroller 30 and outputs a high state signal through resistor R28to the port P14 of the microcontroller 30. Once the ring gear 45 isactuated and reaches the high speed position, the pole contacts thecontact H1 and leaves the contact L1 open. In this case, themicrocontroller receives the high state signal via port P13 and receivesthe low state signal via port P14. The switch 387 is composed of a pairof metallic contacts H1, L1 positioned in the housing at opposite endsof the linear travel of the connection member 54 and a metallic plate(to function as the pole) attached on the connection member 54. As themetallic plate contacts each of the metallic contacts H1, L1, a lowstate signal will be generated and transmitted to the microcontroller 30via the port P13 or P14 representative of the present speed position.Thus the microcontroller 30 is able to detect the present speedposition. The metallic contacts and plate can be disposed on the ringgear and in the gear case respectively or on the push bar and in thehousing respectively.

The speed mode selector 36 has three signal contacts corresponding toauto mode A, high speed mode H2 and low speed mode L2. The auto A, highspeed H2 and low speed L2 contacts are connected to the ports P24, 25,26 of the microcontroller 30 via resistors R16, R15, R14 respectively.Similar to the switch 387 of the load feed back circuit 34, if any ofthe three contacts A, H2, L2 is closed, the microcontroller 30 willreceive a low state signal and identify which port inputs the signal.Thus the microcontroller 30 can know which mode is selected and thenexecute an algorithm for the selected mode as described hereinafter.

The signal generator 35 includes two activation switches S2, S3 arrangedin parallel and is connected to the port P23 via resistor R117. Ifeither of the two switches S2, S3 is activated, an interrupt electricsignal will be generated and outputted to the microcontroller 30 via theport P23. The selected operation then interrupts the currentlyprocessing program and executes a one touch speed variation algorithmwhich will be described hereinafter.

Referring to FIG. 21, once the operator has actuated the trigger button131, the main circuit is closed (step 711). A reset signal is theninputted through port P1 to the microcontroller to initialize themicrocontroller (step 712). The microcontroller 30 then samples thevoltage signal of the battery pack through port 3 to judge whether ornot the battery pack has been over-discharged (step 713). If yes, themicrocontroller 30 will not proceed. If no, the microcontroller 30 thendetects which speed mode is currently selected by reading the signalstate at ports P24, P25, P26 (step 714). When the microcontroller 30detects that the auto mode is selected, it judges whether or not thepush bar 535 of the solenoid 53 is located at the high speed position byreading the signal state at ports P13, P14. If yes, the microcontroller30 proceeds to step 721. If no, the microcontroller 30 controls theH-bridge circuit 33 to vary the direction of electric current applied tothe solenoid 53 for switching the movable member 45 to the high speedposition and then proceeds to step 721. When it is detected that thehigh speed mode is selected, the microcontroller 30 further detectswhether or not the solenoid is located at the high speed position (step716). If yes, the microcontroller 30 proceeds to step 721. If no, themicrocontroller 30 controls the movable member 45 to switch to the highspeed position (step 719) and then proceeds to step 721. In a similarmanner, when it is detected that the low speed mode is selected, themicrocontroller 30 further detects whether or not the solenoid 53 islocated at the low speed position (step 717). If yes, themicrocontroller 30 proceeds to step 721. If no, the microcontroller 30controls the solenoid 53 to switch the movable member 45 to the lowspeed position (step 720) and then proceeds to step 721.

At step 721, the microcontroller 30 controls the LEDs D2 to D5 to showcurrent residual capacity level of the battery pack. The microcontroller30 monitors the voltage across the battery pack 1 in real time (step722) and cuts off the power supplied to the motor 2 when the batterypack 1 is over-discharged. The microcontroller 30 then detects thesignal from the potentiometer 13 indicative of the depression degree ofthe trigger button 131 (step 722) and outputs the PWM control signal tocontrol the rotational speed of the motor 2 (step 723). As the motor 2runs, the microcontroller 30 monitors the current flow through the motor2 (step 724).

The microcontroller 30 then detects the speed mode again (step 726). Ifit is detected that the present mode is the high or low speed mode, themicrocontroller 30 then judges whether or not the motor current exceeds90 A for 500 ms (step 727 or 729). If yes, the motor 2 may have stalledthereby causing the motor current to increase sharply. Thus the powersupplied to the motor 2 is cut off (step 728 or 730). If no, the programreturns to step 721. If it is detected that the present mode is automode, the microcontroller 30 then judges whether or not the motorcurrent exceeds 90 A for 500 ms (step 727 or 729). If yes, the motor 2may have stalled, and the microcontroller 30 executes the automaticspeed variation steps which will be discussed hereinafter. If no, themicrocontroller 30 further judges whether or not the motor currentexceeds 30 A for 500 ms (step 732). If yes, the microcontroller 30executes the automatic speed variation steps. If no, the program returnsto step 721. Step 732 is conducted to judge whether or not the motorcurrent value reaches the switching point where the output efficiencycurve of the power tool 9 at high speed and that at low speed intersect.Thus, if the motor current value equals or is larger than apredetermined value for a preset time period, the automatic speedvariation is activated. It will be apparent that the automatic speedvariation can also be activated immediately as the motor current valueequals the predetermined value.

The automatic speed variation steps include steps 733 to 736. In orderto avoid gear clash when the ring gear 4 5 moves from the high speedposition to the lower speed position, the rotational speed of the ringgear 45 is reduced (preferably to zero). In this case, themicrocontroller 30 temporarily interrupts the power supplied to themotor (step 733). For example, the port P12 is blocked from outputtingthe PWM control signal thereby reducing the rotational speed of themovable member 45 to zero. The microcontroller 30 then controls thesolenoid 53 to move the movable member 45 to the low speed position. Asdescribed previously, the microcontroller 30 may control the H-bridgecircuit 33 to apply a forward electric current to the solenoid (ie toclose the input ports A, B and open the input ports C, D). Thus the pushbar 534 of the solenoid 53 is actuated to move from the high speedposition to the low speed position (step 734). As the movable member 45reaches the low speed position, the position feed back switch S5 isactivated and generates a low state signal output to the microcontroller30 through port 13. The microcontroller 30 then soft starts the motor(step 735). In this case, the microcontroller may vary the duty cycle ofthe PWM control signal to step up the voltage applied to the motor untilthe rotational speed of the motor returns to a normal state. After that,an LS flag is set to indicate current speed position (step 736) and thenthe program returns to step 721. A time interval is required for thering gear 45 to move from the high speed position to the low speedposition. Thus the solenoid can be activated prior to pausing the motor.In this case, the rotational power of the motor is removed before thering gear meshes the rotationally fixed structure. By this means, therotational speed of the ring gear is not reduced to zero. Since there isno driving power applied to the ring gear, the ring gear is able toengage with the rotationally fixed structure smoothly and come to astandstill quickly.

Referring to FIG. 22, as the port 23 of the microcontroller 30 receivesan interrupt signal generated by the signal generator 35, themicrocontroller 30 interrupts the current processing program and turnsto one touch speed variation program (step 751). Initially, themicrocontroller 30 reads the motor current value via port P21 and judgeswhether or not the motor current exceeds a preset threshold value suchas 98 A (step 752). If yes, the motor 22 may be stalled. Themicrocontroller 30 then controls the MOSFET drive circuit 31 tointerrupt the power supplied to the motor 22 (step 753). If no, themicrocontroller 30 then detects what current speed mode is (step 754).If the current speed mode is the auto mode, the microcontroller 30 endsthe present program. If the current mode is the high speed mode or thelow speed mode, a judgment is made on whether a load is applied to thepower tool. This is because when a load is applied, the motor speed isreduced faster than when no load is applied. In other words, the motorwith no load needs more time to stop. To detect whether a load isapplied, the microcontroller 30 judges whether or not the motor currentexceeds 30 A for 500 ms (steps 755 or 761). If yes, there is no loadapplied and the microcontroller 30 pauses the motor 22 and waits 0.5 s(steps 756 or 762). If no, there is a load applied and themicrocontroller 30 then pauses the motor 22 and waits 2 s (steps 757 or763). The microcontroller 30 then controls the solenoid 53 to move themovable member 45 to the low speed position (steps 758) or the highspeed position (steps 764), starts the motor 22 again (steps 759 or 765)and sets a LS or HS flag to indicate present speed position (steps 760or 766). After that, the microcontroller 30 ends the current program andreturns to the main program.

FIGS. 23 a, 23 b and 24 show a seventh embodiment of a power tool 9 ofthe present invention and its driving mechanism 5. The driving mechanism5 includes a small motor 55, a small gear transmission mechanism 56having an output spindle 561 for outputting rotary power of the smallmotor 55, a gear 57 mounted on the output spindle 561 and a circularsleeve 58 rotatably mounted on the outside of the gear casing 47. Thecircular sleeve 58 has a pair of diametrically opposed grooves definedin the circumferential wall for respectively receiving and guidingopposite distal ends of a steel wire 513 and external teeth 584 formedon the circumferential surface. Each groove has first and secondsections 581, 583 substantially perpendicular to the longitudinal axis(the broken lines in FIGS. 23 a and 23 b) of the power drill 9 and athird section 582 disposed between the first and the second sections581, 583 extending obliquely to the longitudinal axis.

FIGS. 23 a and 23 b show the power tool 9 in a high speed and low speedstate respectively. On start up of the small motor 55, the gear 57mounted on the output spindle 561 attains a rotational speed andactuates the sleeve 58 to rotate by engaging the external teeth 584. Inthis case, the steel wire 513 is urged by the third section 582 of thegroove to move in the longitudinal direction and the movable member 45moves with the steel wire 513 from one speed position to the other toachieve automatic speed variation. The small motor 55 is electricallyconnected to an H-bridge circuit controlled by a microcontroller asdescribed hereinbefore so that it can rotate in the opposite directionas the direction of the current is changed. The small gear transmissionmechanism can be omitted if the direct output speed of the small motor55 is relatively low.

In the present invention, the structures of the gear transmissionmechanism and the movable member are not limited to those described inthe above mentioned embodiments. There are various types of conventionalgear transmission mechanism (see for example U.S. Pat. No. 6,796,921) towhich the present invention may be applicable.

1. A power tool capable of variable output speed comprising: a housing;a motor contained in the housing for outputting rotary power; an outputshaft; a gear transmission mechanism disposed between the motor and theoutput shaft for transmission of the rotary power of the motor to theoutput shaft at each of a plurality of gear reduction ratios; a signalgenerator; and a control system electrically coupled to the signalgenerator and operatively engaged with the gear transmission mechanismsuch that when the signal generator is manually activated, an electricsignal is generated and transmitted to the control system to cause thegear transmission mechanism to vary the gear reduction ratio.
 2. Thepower tool according to claim 1, wherein the signal generator comprisesa switch mounted on the housing.
 3. The power tool according to claim 2,further comprising a handle coupled to the housing, wherein the handleis arranged at a predetermined distance from the switch such that theswitch is operable by the hand of the user without removing the handfrom the handle.
 4. The power tool according to claim 2, wherein thecontrol system comprises: a control unit electrically connected to theswitch to receive the electrical signal when the switch is activated;and a driving mechanism actuated by the control unit in response to theelectrical signal to cause the gear transmission mechanism to vary thegear reduction ratio.
 5. The power tool according to claim 4, whereinthe gear transmission mechanism comprises: at least one gear train; anda movable member movable between a first position corresponding to a lowgear reduction ratio and a second position corresponding to a high gearreduction ratio whereby to vary the gear reduction ratio.
 6. The powertool according to claim 5, wherein the gear train is a planetary geartrain which includes a plurality of planet gears, an adjacent planetcarrier, and a rotationally fixed structure immovably associated withthe housing.
 7. The power tool according to claim 6, wherein the movablemember comprises a ring gear which when located at the first positionengages the planet gears and the planet carrier and when located at thesecond position engages the planet gears and the rotationally fixedstructure.
 8. The power tool according to claim 4, wherein the drivingmechanism is electromagnetically actuatable and operatively engages themovable member, wherein the control unit is operative to apply electriccurrent to the driving mechanism to drive the movable member.
 9. Thepower tool according to claim 8, wherein the electromagneticallyactuatable driving mechanism comprises: a solenoid which includes acoil; an iron core linearly movable through the coil; and a push barattached to the iron core and connected to the movable member.
 10. Thepower tool according to claim 8, wherein the control system comprises anH-bridge circuit for varying the direction of the electric currentapplied to the electromagnetically actuatable drive mechanism.
 11. Thepower tool according to claim 4, wherein the control unit is capable ofdetermining whether there is a load applied to the power tool beforeactuating the driving mechanism.
 12. The power tool according to claim11, wherein the control unit pauses the motor for a first predeterminedtime interval if there is a load applied, and pauses the motor for asecond predetermined time interval if there is no load applied, whereinthe first predetermined time interval is shorter than the secondpredetermined time interval.
 13. A power tool for varying output speed,comprising: a housing; a motor contained in the housing for outputtingrotary power; an output shaft; a gear transmission mechanism disposedbetween the motor and the output shaft for transmission of the rotarypower of the motor to the output shaft at each of a plurality of gearreduction ratios including a first gear reduction ratio and a secondgear reduction ratio, the gear transmission mechanism comprising: atleast one gear train; and a movable member variably engaged with thegear train, wherein the movable member is movable between a firstposition corresponding to the first gear reduction ratio and a secondposition corresponding to the second gear reduction ratio; and a controlsystem comprising a mechanism for driving the movable member to movefrom the first position to the second position and from the secondposition to the first position.
 14. The power tool according to claim13, wherein the mechanism comprises a bi-directional keep solenoidcapable of retaining the movable member at the first or the secondposition without electricity.
 15. The power tool according to claim 14,wherein the bi-directional keep solenoid comprises a push bar connectedto the movable member.
 16. The power tool according to claim 15, whereinthe bi-directional keep solenoid further comprises: a longitudinalmetallic shell; a pair of coils arranged longitudinally in the shell; apermanent magnet disposed between the pair of coils; and an iron corelinearly movable through the pair of coils, wherein the push bar isfixed to the iron core.
 17. The power tool according to claim 13,wherein the gear train is a planetary gear train which includes aplurality of planet gears, an adjacent planet carrier, and arotationally fixed structure immovably associated with the housing. 18.The power tool according to claim 17, wherein the movable membercomprises a ring gear which when located at the first position engagesthe planet gears and the planet carrier and when located at the secondposition engages the planet gears and the rotationally fixed structure.19. The power tool according to claim 13, wherein the control systemfurther comprises a control unit operative to apply electric current tothe mechanism to drive the movable member.
 20. The power tool accordingto claim 19, wherein the control system comprises an H-bridge circuitfor changing the direction of the electric current applied to themechanism.